Electrochemical detection of phosgene in gas mixtures

ABSTRACT

An electrochemical cell having a polarographic device and preferably containing formamide as electrolyte is suitable for detecting very small quantities of phosgene in a gas mixture. Water, sodium acetate and thickener may be added to the electrolyte.

This is a division, of application Ser. No. 804,696 filed June 8, 1977,now U.S. Pat. No. 4,200,512.

This invention relates to a polarographic process for the measurement ofphosgene in a gas mixture, in particular for the detection of traces ofphosgene in air, and to an apparatus for carrying out this process.

Polarographic arrangements for detecting gases are already known. Theyare relatively uncomplicated and reliable in operation. The quantitativemethod of detection is particularly important for environmentalprotection. The physical changes and changes in composition should bemeasured as far as possible continuously. A gas detector operating bythe polarographic process usually contains a cell with three electrodes.The stability and sensitivity of such a cell are higher than that of atwo-electrode arrangement, as is described in GermanOffenlegungsschriften Nos. 2,155,935 and 2,354,149, but thispolarographic method of measurement has the disadvantage of not beingsufficiently selective. In many cases it cannot be foreseen what gasesare liable to occur in a gas mixture, but it is generally necessary todetermine the concentration of a particular gas. The presence of oxidesof nitrogen, for example, simulates an excessively high carbon monoxidecontent, while the sensitivity of detection of phosgene is affected byvarious constituents such as sulphur dioxide, gases which form chlorineions, and others.

In German Offenlegungsschrift No. 2,155,935 it is mentioned that ifseveral gases are present in a gas mixture, measures must be providedfor detecting and determining the various impurities separately. It issaid that this may be achieved by passing the air sample successivelythrough individual cells in each of which only one impurity reacts whilethe other impurities are inert. The anode and cathode materials,electrolyte and temperature of the electrolyte should be suitably chosenfor this purpose. The said Offenlegungsschrift contains no indication asto how phosgene could be detected in small quantities in a gas mixture.

An apparatus which is said to be suitable for detecting hydrocyanicacid, hydrogen sulphide and chlorine gas is available on the market. Itselectrolyte must be continuously renewed and its sensitivity ofdetection is affected by the presence of other gases, for examplesulphur dioxide. This apparatus is not suitable for the detection ofphosgene.

It is an object of this invention to provide a reliable, highlysensitive and convenient gas detector for phosgene.

The problem is reduced or substantially solved by a polarographicprocess in which phosgene reacts with the electrolyte and thepolarographic detection of the reaction product is not affected by othergases in the gas mixture.

A particularly suitable electrolyte for this process is formamide, whichreacts with phosgene according to the equation:

    COCl.sub.2 +HCO--NH.sub.2 =HCN+CO.sub.2 +2 HCl,

so that HCN is detected selectively.

One particular advantage of this arrangement is the high sensitivity ofmeasurement for HCN, which is not deleteriously affected by the presenceof other gases such as carbon monoxide, sulphur dioxide, gaseoushydrogen chloride or oxides of nitrogen.

The electrolyte may contain water and sodium acetate. In particular, thesensitivity is improved by the presence of from 0 to 15%, preferably10%, by weight of water and from 5 to 20%, preferably 20%, by weight ofsodium acetate. The electrolyte may be thickened by means of unreactivesubstances, for example methyl cellulose. Continuous renewal of theelectrolyte is unnecessary. The structure of the gas detector istherefore very compact.

An example of the invention is described below with reference to thedrawing.

The auxiliary electrode 3 is placed in a screw thread 2 cut into acylindrical body of polypropylene 1. A mesh wrapped round the threadedpart of the cylinder 1 serves as working electrode 4. The electrolyte 5between the auxiliary electrode 3 and working electrode 4 is thickened.An electrolyte-permeable foil 6 placed on the inside of the workingelectrode 4 serves both to stabilise the electrolyte mechanically and toreduce the effective quantity of electrolyte, thereby improving the timefactor. The reference electrode 7 is placed centrally and dips into theelectrolyte 8 which has the same composition as electrolyte 5 butwithout the thickener. Electrolyte 8 is electrically connected to thethickened electrolyte 5 channels 9 between the working electrode 4 andauxiliary electrode 3. Each channel 9 contains a diaphragm 10 whichprevents the outflow of electrolyte 8 but permits fresh electrolyte tobe supplied through it so that the external electrolyte 5, which isthickened, maintains its composition for a long time. The voltage of thepolarographic stage of the gas which is to be detached is preselected onthe potentiostat 11. Instrument 12 indicates a current proportional tothe concentration. The apparatus 1 is enclosed in a housing 13 providedwith a gas inlet 14 and gas outlet 15. An annular frit 16 serves toretain the gas to be measured and, at the same time, allows gas to flowuniformly through the chamber 17.

What we claim is:
 1. In a process for the polarographic measurement ofphosgene in a gas mixture wherein the improvement comprises providing apolarographic cell containing at least two electrodes and an electrolytein the cell containing formamide; reacting phosgene in a gas mixturewith the formamide to produce a reaction product including HCN; andpolarographically measuring the HCN to obtain a current which isproportional to the concentration of phosgene and is unaffected by theother gases in the gas mixture.
 2. The process according to claim 1,comprising providing the electrolyte with up to 15% by weight of water.3. The process according to claim 2, comprising providing theelectrolyte with 10% by weight of water.
 4. The process according toclaim 2, further comprising providing the electrolyte with from 5 to 20%by weight of sodium acetate.
 5. The process according to claim 4 furthercomprising providing the electrolyte with 20% by weight of sodiumacetate.
 6. The process according to claim 4, further comprisingproviding the electrolyte thickened with unreactive substances.
 7. Theprocess according to claim 6, further Comprising providing theelectrolyte thickened with methyl cellulose.
 8. The process according toclaim 1, further comprising providing the electrolyte with 10% by weightof water, 20% by weight of sodium acetate and a methyl cellulosethickener.